Testing for carbonyls

Question 1

,Main test to see if a C=O bond is present

Answer 1

2,4-DNPH test

Question 2

What does 2,4-DNPH have a positive test for?

Answer 2

Both aldehydes and ketones

Question 3

Conditions for 2,4-DNPH test

Answer 3

Concentrated sulfuric acid
dissolved in methanol
Warmed

Question 4

Positive result for 2,4-DNPH test

Answer 4

A bright orange precipitate is formed which are crystals

Question 5

What can be done after the 2,4-DNPH test?

Answer 5

Purify crystals by recrystallisation
Complete melting point test to find MP
Compare to data booklet values of MP to identify the carbonyl we had

Question 6

Does the 2,4-DNPH test differentiate aldehydes from ketones?

Answer 6

NO after a positive result for 2,4-DNPH test we can complete after tests to differentiate

Question 7

3 tests that give a positive result for aldehydes but NOT ketones

Answer 7

Tollens’ reagent
Fehling’s/ Benedict’s solution
Acidified dichromate (VI) ions

Question 8

Why can the 3 tests distinguish an aldehyde from ketone by giving a positive result for the aldehyde?

Answer 8

Because aldehydes can be oxidised again to carboxylic acids whereas ketones can only be oxidised by breaking C-C bonds
Positive result = shows substance was oxidised (aldehyde)

Question 9

How to represent oxidising agent?

Answer 9

[O]

Question 10

What is Tollens’ reagent?

Answer 10

Colourless silver nitrate solution dissolved in aqueous ammonia
Complex ion [Ag(NH3)2]+

Question 11

Another name for Tollen’s reagent test

Answer 11

Silver mirror test

Question 12

Conditions for Tollens’ reagent test

Answer 12

Heat the silver nitrate solution in aqueous ammonia with an aldehyde

Question 13

Oxidising agent in Tollens’ reagent

Answer 13

Ag+ ions are reduced to Ag
And oxidises the aldehyde

Question 14

What are aldehydes oxidised to when testing for them?

Answer 14

Carboxylic acids

Question 15

What observation indicates a positive result for Tollens’ reagent?

Answer 15

A silver mirror formed which coats test tube as Ag+ ions were reduced to Ag atoms

Question 16

What is Fehling’s solution?

Answer 16

Cu 2+ ions dissolved in NaOH

Question 17

What is Benedict’s solution?

Answer 17

Cu 2+ ions dissolved in Na2CO3 solution

Question 18

Oxidising agent in Benedict’s/Fehling’s solution

Answer 18

Cu 2+ ions that are reduced to Cu2O (Cu 1+ ions)

Question 19

Observation for positive result of Benedict’s/Fehling’s test

Answer 19

Colour change blue solution to form red precipitate of copper oxide

Question 20

Word equation for Fehling’s/Benedict’s?

Answer 20

Aldehyde + [O] ——-> Carboxylic acid + copper oxide

Question 21

What is Acidified dichromate ions?

Answer 21

H+/Cr2O7 2- ions

Question 22

Oxidising agent for acidified dichromate test?

Answer 22

Cr2O7 2-
The Cr 6+ ions are reduced to Cr 3+ ions

Question 23

Observation for positive result acidified dichromate ions test?

Answer 23

Colour change orange to green

Question 24

Iodoform test reagent

Answer 24

Iodine in presence of cold aqueous hydroxide alkali

Question 25

What does iodoform give a positive result for?

Answer 25

Methyl carbonyl groups, the carbonyl group is bonded to a methyl group which is CH3 O=C-CH3

Question 26

What molecules give a positive result for the iodoform test?

Answer 26

Ethanal Any ketone which C=O is bonded to the 2nd carbon so there is a methyl group to be the end of the chain

Question 27

Positive result for iodoform test

Answer 27

Yellow precipitate of CH3I And antiseptic smell

Question 28

Why do only methyl carbonyl groups give a positive iodoform test result?

Answer 28

Because a yellow precipitate of CH3I was formed

Question 29

How to prepare Tollen's reagents?

Answer 29

Mix aqueous silver nitrate and aqueous sodium hydroxide Add aqueous ammonia drop by drop until the silver oxide dissolves completely

Question 30

Tollen's reagent structure